Molten metal filter and method for making same

ABSTRACT

A porous ceramic body of high structural strength and integrity is disclosed, along with the method of fabricating such, whereby a curable resin and a sinterable ceramic are mixed and then admixed to removable pore formers, then consolidated into a green body, the pore formers removed, and sintered into the porous ceramic body.

This is a File Wrapper continuation of co-pending application Ser. No.06/879,789 filed Jun. 27, 1986 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of filtering contaminantsfrom molten, or liquid phase metals and particularly to a means forseparating non-metallic inclusions and contaminants from molten metal asit is flowed.

2. Background

In the melting, refining and forming of metals, typically when moltenmetals are cast, it is desirable to separate exogenous intermetallicinclusions from the molten metal. Such inclusions result, in moltenmetals, from impurities included in the raw materials used to form themelt, from slag, dross and oxides which form on the surface of the melt,and from small fragments of the refractory materials that are used toform the chamber or vessel in Which the molten metal melt is formed.Such inclusions, if not removed from the molten state of the metal, canresult in weakened points and/or porosity in the final formed andsolidified metal body which is the eventual downstream end product ofthe melting operation.

Typically, in a metal casting operation, the metal melt is formed in afurnace wherein the constituent components are added in the form ofunmelted scrap and/or refined virgin metal, deoxidizing agents invarious forms (both solid and gaseous or a combination of both) andalloying elements. Very light (less dense) solids and gases tend tomigrate to the surface of the melt where they either effervesce or floatin combination with partially and completely solidified oxides knownvariously as slag and dross. The higher density impurities in the melttend to remain in some degree of suspension in the liquid phase of themetal, or melt, as the fluid flow convection currents are generatedwithin that melt by the heating means applied by the melting furnace.

In the melting operation, the furnace acts as a vessel to hold themolten metal as it is being melted and, depending on the particularspecies of molten metal or alloy being formed, for a period of timefollowing melting, to refine the molten metal by way of the gases andlow density impurities migrating to the surface The molten metal is thentransferred, typically to another vessel, such as, for example, a ladle,to transport it to the forming means, such as, for example, a mold.Alternatively, the molten metal may be drawn directly from the furnaceand flowed by gravity through a channeling means to a forming means,such as a continuous caster. A variety of other methods, used forremoving molten metal from a furnace and conveying it to a formingmeans, are well known to those with skill in the art.

During this transportation or conveyance phase, wherein the molten metalis moved from the melting furnace to the forming means, it is desirableto ensure that the dross or slag, from the surface of the melt, does notbecome included in the formed metal and, also, that the higher density,exogenous intermetallic inclusions in the melt are not included in thatformed metal.

One method that is used to prevent the inclusion of exogenousintermetallic substances, including slag or dross, in the formed metalbody is to filter the molten metal as it is flowed from the meltingfurnace to the forming means. A variety of means for accomplishing thisfiltration are well known to those with skill in the art. Recentexamples of this can be found in U.S. Pat. Nos. 4,444,377; 4,426,287;4,413,813; 4,384,888; 4,330,328; 4,330,327; 4,302,502; 4,298,187;4,258,099; 4,257,810; 4,179,102; 4,159,104, 4,081,371; 4,032,124 and3,869,282. There are many additional recent references readily availablethat demonstrate methods of filtering molten metal. Also, there are manyolder references available, which date further back into history andwhich show apparatus and methods for filtering molten metals, such as,for example, U.S. Pat. No. 3,006,473.

In such systems, a filter medium or filter element is used. The basicmaterial property that is necessary in a filter medium is that it beformed from a high temperature material which will withstand theelevated temperatures of molten metals and be stable in such anenvironment. That is to say that the material must not be subject todeterioration from melting, chemical reactions or erosion at suchelevated temperatures. Also, the filter medium must maintain structuralintegrity at such elevated temperatures. And, of course, to act as afilter, the filter medium must be capable of either entrapping orpreventing the flow of solids, liquids, and semi-liquids, all of whichare non-metallic or intermetallic, either by chemically reacting thefilter medium material with such inclusions and/or by mechanicallypreventing the flow thereof through the filter medium, while stillpermitting and facilitating the flow of the molten (liquid) metaltherethrough. Further, such filter media are used in productionfacilities in association with unskilled or semi-skilled labor and heavyindustrial machinery, equipment and tooling. Thus, such filter mediashould exhibit a high degree of structural integrity, at roomtemperature, such that rough handling will not be detrimental.

Many different designs of filter medium are known to those with skill inthe field. Also well known are the uses of many different materials for,and many different methods of fabricating, or producing, porous bodieswhich can be used as filters. U.S. Pat. No. 3,796,657, for example,teaches the use of a fluidized and sintered aggregate of particles toform a porous chromatographic filter medium for separating gases fromliquids and different liquids from each other. U.S. Pat. No. 4,430,294,as another example, teaches the formation of porous nickel bodies byusing reducing gases and carbon powder to form interstices in nickelpowder, during a rapid sintering process. U.S. Pat. No. 4,285,828, asyet another example, teaches forming a porous aluminum body by combiningaluminum powder with an expanding agent, such as a fine salt, hotpressing the mixture and dissolving the expanding agent from the poresof the body. U.S. Pat. No. 4,391,918, as yet another example, shows theimpregnation of an open celled organic foam with a slurry composedmostly of aluminum oxide plus sintering aids. The organic foam is thenburned out as the slurry is sintered to form an aluminum oxide ceramicfoam which can be used to filter molten metals.

Many older patents teach the bonding of crystalline ceramic material,such as silicon carbide or alumina, with a vitrified ceramic materialsuch as glass. Such is taught, for example, by U.S. Pat. No 2,007,053.Also, it is known to directly sinter particles of ceramic material intoa porous body to form a filter medium. Such is taught, for example, byU.S. Pat. No. 2,021,520. Finally, it is known to mix a ceramic materialsuch as aluminum oxide with a combustible material such as carbon andburn out the combustible material during sintering to produce a porousbody. Such is shown, for example, in U.S. Pat. Nos. 2,360,929 and2,752,258.

One of the problems that is inherent in many of the filter media whichare useful for filtering molten metals is that it is difficult to renderthe pores or passageways throughout the filter media substantially openbut also controlled in sizing such that the molten metal will freelyflow through the filter media cross section at a controlled rate and sothat all solid matter, of a calculated size range or larger, will beuniformly blocked from passage through the full cross section ofthickness of that filter media.

Another problem that is inherent in many of the known filter media whichare useful for filtering molten metals is that the surfaces of the poresor passageways through the filter media are not smooth, and thus, aresusceptible to non-uniform build up of solid matter which tends toadhere more readily to the non-smooth surfaces adjacent to the entryside of the filter medium, thus not fully utilizing the thickness ofthat filter medium to fully trap such solids. Further, non-smoothsurfaces tend to create turbulence in the flow of molten metal, thusinhibiting the smooth flow thereof. These phenomena shorten the usefullife of the filter medium as the flow of molten metal therethroughdecreases at a relatively greater rate than if the full thickness of thefilter media were usable to trap the solid matter.

Another problem that is inherent in many of the known filter media isthat it is difficult to localize sizes of pores or passageways throughthe filter media to form pore size gradations either through the filtermedia or from one side to another across the face thereof, when desired.Such gradations are useful in specialized situations for preventing thepassage of mixed solid materials of various types, and enhancing theseparation of certain gases from the molten metal, as it flows throughthe filter media. Such gradations may also be used to selectivelycontrol flow rates in specialized circumstances.

Yet another problem that is inherent in many of the known filter mediumis that they are too brittle or too friable, or both, at elevatedtemperatures as well as at room temperature. Thus structural failure ofthe known filter media has been a major problem related to the economicsof using such for filtering molten metal. The strength is known to bediminished by the presence of sharp corners, non-continuous ceramicstructure, and large pores in the load bearing sections of the ceramicmaterial. For example, filamentary pores, left behind after theformation of reticulated foam filter media, exhibit such defects.

The present invention provides a filter medium, and a means forproducing it, without filamentary pores, but with relatively uniformcell sizes, with passageways or pores therebetween, with relativelysmooth surfaces on the cell walls and the walls of the passageways orpores therebetween, with the edges, or discrete transition areas,between cells and their interconnecting pores, being rounded off orsmoothed, and a means for forming a filter medium with cell sizegradations or localization of cell sizes or passageway (pore) sizes.Also, the present invention provides a filter medium with a high degreeof structural integrity both at room temperatures and at the elevatedtemperatures associated with molten metals.

SUMMARY OF THE INVENTION

The present invention includes a method of making a porous ceramic bodyas well as the porous ceramic body itself A quantity of bondable orsinterable ceramic powder, or mixtures of different ceramic powderswhich will either bond or sinter together, are thoroughly mixed with apolymer binder. A pore former is also mixed with the powder or powdersand a polymer binder. The resultant mixture of the constituentcomponents is then consolidated into a relatively dense self-sustainingbody. The pore former can be any suitable solid that can be removed byleaching, melting or pyrolysis. The pore former may include therewith alubricant which is not soluble in the polymer binder and which does nothave deleterious effects on the curing of the polymer binder. The poreformer, on the other hand, may have inherent self-lubricatingproperties. Or, alternatively, the resin may by itself, or incombination with other materials, provide lubricity between the poreformer surfaces and the resin. The pore former may, also, be deformableunder pressing conditions. The polymer binder may be any thermosettingor thermoplastic resin that can be readily mixed with ceramic powder orpowders and the pore former, provided that the polymer binder does notdissolve or dilute the lubricant, used in association with the poreformer, therein. The self-sustaining body is then consolidated to ashaped size, for example, pressure compaction by die pressing. Thepolymer binder is then cured. Then the pore former is removed, and theshaped body is elevated to sintering temperature to form a poroussintered ceramic body with cells and interconnecting pores therebetween,in a form which is known in the field of geology as "moldic porosity".The sintered ceramic body has smooth walled cells formed by the poreformers, and pores, with rounded edges, which interconnect the cells,the rounded edges forming the transition points between the cells andthe pores. The cell and pore characteristics may be controlled oraffected by the deformability, size, distribution and location of thepore formers, and the type, distribution and amount of lubricant meansused, and also by the type of consolidation process used, thecomposition of the sinterable or bondable ceramic powder, or mixture ofthose ceramic powders, and the type of polymer binder which is used.Either by itself, or in combination with a lubricant, the selectedpolymer binder should be such that it tends to bead in contact with thesurface of the pore former. The ceramic powders, as sintered to form theceramic body, form a continuous, uninterrupted, void-free and densesintered ceramic matrix interspersed between the interconnecting voidsand pores, which exhibits low friability and low brittleness in regardto physical shock. The porous ceramic body of the present invention isuseful for molten metal filtration as well as a variety of otherapplications, such as, for examples, catalyst supports and gasdispersion mechanisms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a molten metal filter formed from aporous ceramic body according to a preferred embodiment of the shape ofthe present invention.

FIG. 2 is a schematic elevational view of the molten metal filter shownin FIG. 1.

FIG. 3 is a schematic representation of a micrograph of a cross sectionof the preferred embodiment of the present invention.

FIG. 4 is a projection view, partially cut-away, of an alternate poreformer within the present invention.

DETAILED DESCRIPTION

According to the present invention, a sintered or bonded porous ceramicbody containing interconnected cells is formed. The cells areinterconnected generally in an open cellular-like mode such thatcircuitous or tortuous pathways are formed through and throughout theceramic body. The cell sizes may be altered, in either a gradation orabruptly, from one section of the body to another. It is also possiblein a given ceramic body, according to the present invention, to formunconnected cells, or no cells at all, in some portions of the body,while other portions contain interconnected cells, thus localizing thepathways through the ceramic body in certain portions or areas thereof,as desired, as will be well understood by those with skill in the fieldon reading the following description. It is also possible in a givenceramic body, according to the present invention to include a range ofsizes and/or shapes of cells such that either the size range isuniformly repeated throughout the ceramic body from one section thereofto another or the cell sizes are randomly arranged and positionedthroughout the body.

The ceramic body of the present invention can be formed and shaped,generally, by any conventional method that is applicable to the shapingand forming of sintered ceramic bodies. FIG. 1 schematically shows apreferred form and shape of the ceramic body 11 which is particularlyuseful in filtering molten metal. FIG. 1 shows a flat plate form ofceramic body 11 in the general shape of a square, however, the shape ofthis preferred embodiment could be rectangular, round, hexagonal,irregular, etc., depending upon the shape of the holding mechanism (notshown) into which the ceramic body is to be inserted for use. Of course,the sizing of the preferred embodiment of the ceramic body shown in FIG.1 would be set to correspond to the size of that holding mechanism andto permit ease of insertion and removal in respect thereto.

The thickness 13 of the ceramic body 11 will depend on the holdingmechanism, on the one hand, but also could depend on other factors suchas the degree of filtration desired, the location of the ceramic body 11in the filtering apparatus into which it is inserted, the frequency ofuse thereof, and/or the location and sizing of the interconnected poresthroughout the ceramic body 11. Typically, the ceramic body 11 would,for example, have a thickness 13, as shown in FIG. 2, of about 2" (5.1cm) and, being generally square, would have, for example, equivalentface dimensions 15, nominally, of about 24" (60.7 cm), 20" (50.8 cm),17" (43.2 cm), 15" (31.8 cm), 12" (30.5 cm), 9" (22.9 cm) or 7" (17.8cm). The angle of bevel 17, as shown in FIG. 2, is typically, forexample, about 17°, however, this also depends on the correspondingsegment of the holding mechanism, associated with the filteringapparatus, with which the bevel angle 17 is to be mated.

In operation, the molten metal is preferably gravity flowed, downwardly,through the ceramic body 11, from entry face 21 to exit face 23, asshown in FIG. 2. Of course, it will be well understood by those withskill in the field that gravity flow is not the only means of flowingmolten metal, as pumps for such are available in the market. Bevel 19functions to both serve as a stationary seat abutment for ceramic body11 and to permit ease of installation and removal thereof from thecorresponding holding mechanism as will be easily understood by thosewith skill in the field. The ceramic body 11 may be used in conjunctionwith a gasket means (not shown) interposed between bevel 19 and thecorresponding mating segment of the holding mechanism, as is known tothose with skill in the field.

Bondable or sinterable ceramic powders, or mixtures of ceramic powderswhich are bondable or sinterable, are used as the starting raw material.The specification of the powder depends on the application in which theporous ceramic body is to be used. For filtration of molten aluminum,for example, alumina (Al₂ O₃) may be used with appropriate sinteringaids, or binders, such as, for example, calcium-alumino-borate glass ora phosphate-alumino-borate glass. Also particularly useful as sinteringaids for alumina are magnesium oxide and/or calcium oxide. A typicalcomposition of a liquid phase sinterable ceramic powder is 97.0 wt.percent Al₂ O₃ with the addition of 3.0 wt. percent of glass powderformed from a combination of CaO, Al₂ O₃ and B₂ O₃ (molarratio=1:0.79:1.31). A typical composition of a solid state phasesinterable ceramic powder is 99.8 wt. percent Al₂ O₃ with 0.2 wt.percent MgO added. An example of an alumina (Al₂ O₃) which can be usedis A16SG which is commercially available from the Aluminum Company ofAmerica (Alcoa). Other equivalent sources are also availablecommercially.

For higher temperature applications, such as the filtering of moltencopper, molten ductile or grey iron, or molten steel, it is preferredthat a sinterable starting powder which is predominantly high purity,submicron partially stabilized zirconia (PSZ), or a mixture of PSZ andspinel powders which are sinterable, be used. Other materials may alsobe used in such high temperature applications or for other applicationsof the porous body. Such materials, for example, are SiC, TiB₂, B₄ C,Si₃ N₄ and SiAlON, all of which can be rendered sinterable and/orbondable as is well known by those with skill in the field. Materialswhich are usable as ceramic powders within the present invention may beeither liquid phase or solid phase sinterable.

The polymer binder that is used may be either a thermosetting or athermoplastic organic binder which can be pyrolyzed at temperaturesbelow the sintering or bonding temperature, as the case may be, of theceramic material. A preferred characteristic of the polymer binder isthat it can be thoroughly and easily mixed with both the ceramic powderand the pore former. Preferably, the polymer binder, which constitutesabout 30 to 80 volume percent of the mixture which contains only thatpolymer binder and the sinterable ceramic powder, will have a viscosityof less than about one million centipoise. This relatively low viscosityespecially facilitates the blending of the polymer binder into theceramic powder and pore former materials with the use of conventionalmixers such as, for example, double arm mixers or conical mixers. Lowviscosity is especially important where the consolidation processincludes the injection molding of a paste of the polymer binder and theceramic powder into a bed of pore former material. In such anapplication, the low viscosity decreases the injection pressure requiredand also the degree of elevated temperature as is normally associatedwith the injection molding process. Especially low viscosity ispreferred for use of the consolidated material in the pour molding orslip casting methods of forming ceramic bodies. Also, it is possible toextrude the consolidated material into various forms.

Appropriate plasticizers can be used with the various types of usablepolymer binders. For example, mineral oil can be blended with eitherhigh or low density polyethylene resins to lower the viscosity, anddibutyl phthalate can be blended with polyester resins with a similareffect. Such plasticizers, when integrated into the resin, may also actas a lubricant and/or may cause or enhance the beading of the resin incontact with the pore former surface.

A variety of different polymer binder materials can be used in thepresent invention. As suggested above, thermoplastic resins such as lowor high density polyethylene are suitable, especially where injectionmolding techniques are used to form the consolidated body. Thermosettingresins such as epoxy or polyester are suitable where die pressingtechniques are used to form the consolidated body. Polyester, inparticular, is suitable, in relatively low viscosity form, for use whereslip casting or pour molding techniques are used to form theconsolidated body. Other types of resins are considered useful, such as,for example, polypropylene, phenolic and polyvinylchloride, providedthey meet the above stated functional criteria. One key criterion inrespect to the resin used is that it must either be liquid state, tobegin with, or it must liquefy during the consolidation process, atleast on its surface. Following this the resin must solidify. Thisphenomenon is referred to herein, variously, as "curing", "cure" and"cured".

One of the primary requirements for the resin, used as the polymerbinder, is that it provide sufficient strength, after curing, to enablehandling of the formed green body and, preferably, machining thereof.Another primary requirement is that the polymer binder maintain thestructural integrity of the green body during the removal of the poreformer materials and to a sufficiently high temperature to ensure thatthe green body does not disintegrate before it is calcined, as will beexplained hereinafter. As used herein, the term "integrity" or"structural integrity" refers to capability of the structure to maintainand sustain itself in the shape and form in which it is produced,without external support, during succeeding manufacturing steps andthereafter in normal handling.

The pore former can be any suitable material that can be readily removedby liquification or other means from the consolidated body. For example,another means of removal of the pore former may be pyrolization ordirect sublimation. One approach to removing the pore former material byliquification is by leaching with a solvent. An example of this is theuse of calcium chloride, CaCl₂, as a pore former, with the leachingdone, simply, with water. Another method of removing the pore former byliquification is by heating the green body to melt out the pore formermaterial. An example of a pore former that can be used in this method iswax.

A critical element in the present invention is that there be apredominant relative difference in the surface tension of the poreformer in respect to the surface tension of the resin. That is to saythat the wetability of the resin in respect to the pore former should besuch that a "beading" effect occurs. Thus, in this respect, the resin,before curing, will tend to bead on those surfaces of the pore formerwhere there is no physical restraint to such; i.e., where there is somediscrete open volume in which the beading can occur. As will bedescribed hereinafter this occurs, within the present invention, in two(2) significant areas: the surfaces of the cells and the surfaces andedges of the interconnecting pores. As used herein, the term "edges" inrespect to "interconnecting pores" or "pores" refers to the point ordiscrete area of transition between the cell walls and the walls of theinterconnecting pores. The beading effect may be micro, for example,where the cell walls are smoothed by this beading, or it may berelatively macro, for example, where the pore edges are rounded offthereby. The wetability differential may be effected naturally as, forexample, where the pore former material naturally produces a beading ofthe resin. Or it may be affected by modifying the composition of, and/orthe surface of, either the pore former or resin, or both, such as forexample, by adding a lubricant to the pore former, or addingplasticizers to the resin, or both. As used herein, the terms"lubricating properties", "lubricant", and "lubricity" are related tothe foregoing beading phenomena as well as to the normal propertiesassociated with those terms in respect to friction reduction. As usedherein, the term "predominantly" means more so, than not, that which isspecified, and the term "substantially" means being largely, but notwholly, that which is specified.

In the preferred embodiment of the present invention, the pore former iseither combined with a lubricant, or acts, itself, as a lubricant. Thelubricant can be soaked into the pore former, coated onto the poreformer or mixed with the pore former material when the pore formers are,themselves, formed. For example, calcium chloride granules being porous,may be soaked in a lubricant such as, for example, No. 2 diesel fueloil. As another example, urea spheres have been made by mixing the ureain No. 2 diesel fuel oil, followed by the formation of small sizedspherical particles by conventional methods. Wax, on the other hand,does not normally need the addition of a lubricant as it tends to actnaturally as a lubricant by itself. These phenomena may also be foundwhere other materials, for example, low melting point metals such aszinc or lead, are used as pore formers. It has been found that by usingeither the lubricating properties of the pore former material, or by thecombination of a lubricant with the pore formers or the pore formermaterials, the wall surfaces of the cells formed are noticeably smootherthan those formed without a lubricating means. In addition, the ease offormation of the green body in Pressing and injection moldingtechniques, is greatly enhanced. That is to say that the pressurerequirements of consolidation, where pressure is used, are significantlyreduced.

In addition, or alternatively, the lubricant, upon heating, may generateenough vapor pressure to puncture a thin film of ceramic and polymerwhich is interposed between adjacent particles of pore former. This willaid in generation of connected porosity. The properties of the lubricantshould preferably be such that they allow lubricated pore formerparticles to contact each other, Without any substantial interveningmixture of ceramic and polymer binder at those points where adjacentparticles of pore former are in close proximity to each other. Thisdisplacement of the ceramic and resin binder is preferred to enhanceconnected porosity.

Typically, pore formers in the range of +3.5-4, +4-6, +6-8 and +8-20, asrelated to Tyler mesh screen sizing, are suitable for forming pores inthe size range of about 500 to about 1300 microns. These pore formersize ranges are specifically useful in a substantially alumina sinteredceramic body for filtering molten aluminum. A preferred pore formermaterial for forming the foregoing sized pores in a sintered aluminabody, where leaching is appropriate to remove the pore former, iscalcium chloride. Preferably, the calcium chloride is sieved through aseries of Tyler mesh screens, as is well known to those with skill inthe field, and used, either monosized or multisized, as desired. Areadily available and acceptable form of calcium chloride which may beused as a pore former is calcium chloride ice melting material availableat most hardware stores for home use, etc. In all pore formers, agenerally spherical shape is more preferred, although other shapes whichwill permit the formation of smooth walled, interconnected cells, areequally acceptable. The surface of the pore formers as used shouldexhibit a smoothness of about 125 micro inches or smoother, preferably32 micro inches or smoother for best results. This can be achievedeither by employing pore formers with these surface characteristics orby the surface of the pore formers being smoothed by the adherence ofliquid during the consolidation process.

An alternate shape of a pore former is shown in FIG. 4. The pore formerof FIG. 4 has been formed from a solid, rectangular shaped block of wax41. The forming has been done by any appropriate means or method as willbe well understood by those well skilled in the field. The forming is inthe nature of grooves 43 extending half way through the depth 45 of theblock 41 from face 49. Alternate grooves 47, set at a 90° angles(perpendicular to grooves 43, extend half way through the depth 45 ofthe block 41 from face 51 which is that face which is opposite to, buton a parallel plane with, face 49. At the point where the depthextension of grooves 43 and alternate grooves 47 meet, windows 53 areformed. As will be well understood by those with skill in the art, thepore formers of FIG. 4 may be stacked upon each other with alternategrooves 47 being positioned perpendicular to or otherwise at angle togrooves 43 of the block 41 beneath. Also windows 53 of each block 41 maybe aligned or non-aligned with the windows 53 of the block 41 beneath.The pore former shown in FIG. 4 is particularly advantageous where afilter medium of zirconia is to be formed for the filtration of moltenferrous metals such as, for example, steel.

As mentioned above, the mixing of the ceramic powders and resincomponents of the system can be done in any conventional manner. It is,however, important that those components be uniformly mixed and equallyand evenly dispersed among each other to ensure that the sinterableceramic powder is evenly dispersed in the polymer binders. When themixture of these components is subsequently blended and mixed with thepore formers, it is important to ensure that there are no voids thereinand that all interstitial space between the pore formers issubstantially filled to ensure structural strength and integrity.

In a situation where localization, gradation or disruption in theuniform dispersion of the cells (as previously filled by the poreformers) is desired in the sintered porous body, the pore formers areadded selectively to the mixture. For example, a uniform mixture ofsinterable ceramic powder and polymer binder are formed and divided intotwo (2) portions. Into the first portion, a given size of pore formersare uniformly mixed and the mixture is placed into the die of a press,partially filling the die cavity, and leveled. The second portion of thesinterable ceramic powder/polymer binder mixture is mixed with adifferent sized pore former and is placed into the same die cavity, ontop of the first mixture, thus filling the die cavity to the appropriatelevel for pressing. The sintered porous ceramic body resulting from thismethod will have localized cell sizing, with two distinctly differentcell sizes in two different sections of that body, respectively. Thismethod can be varied to add additional cell sizes to localized sectionsof a given body, or to decrease or increase the number of cells for agiven volume at localized sections of that body. Also, it is possible toeliminate the cells in discrete portions or sections of that body, aswill be well understood by those with skill in the field.

A mixture of Alcoa A16SG alumina was blended with Silmar S-585 polyesterresin, as manufactured and marketed by The Standard Oil Company and 0.5wt. percent of methyl ethyl ketone (MEK) peroxide, manufactured byPennwalt/Ludicol, as a curing catalyst for the polyester resin. Thealumina constituted about 40 vol. percent of the mixture of alumina andpolyester resin. Added to the mixture of these first two components wascalcium chloride of a +4-6 Tyler mesh sizing. The calcium chloridecomprised about 70 vol. percent of the final mixture of the threecomponents. The mixture was thoroughly blended in a standard double armmixer and several portions were subsequently loaded into an 21/4" (5.7cm) round die, with a 1" (2.5 cm) working pressures in a range of 2400to 2500 psi were adequate to ensure that the calcium chloride granuleswere sufficiently in contact with each other to produce the formation ofsubstantially interconnected pores in the sintered porous ceramic bodyproduced. As used herein, the term "cells" refers to the discrete openvolumes, or voids, within the porous ceramic body formerly occupied bythe pore formers and the term "pores" refers to the interconnectingpassageways between the cells. This polyester resin was selected for usewith calcium chloride because it was noted that it appeared to have arelatively low amount of wetability in respect to calcium chloride. Thatis to say that it appeared to bead up more when in contact with thesurface of a calcium chloride pore former in comparison to otherpolyester resins which were considered.

It has been found that the pressure required to consolidate the greenbodies being formed can be significantly reduced if a lubricant is addedto the pore former or if the pore former is self-lubricating, as is thesituation where wax pore formers are used. This reduction in pressure issignificant, being about 1/3 or less of that otherwise required. In thepress and die compaction forming technique, the typical 2400 psi to 2500psi pressure, normally required to compact the mixtures to the pointwhere the sintered porous body contained cells that were substantiallyinterconnecting, was capable of being reduced to a typical range ofabout 700 psi to 800 psi. This was done by impregnating the porouscalcium chloride with No. 2 diesel fuel oil. Another benefit accrued, byusing the lubricant, is that the walls of the cells of the sinteredporous body were visibly and notably smoothened. Further, it was foundthat the edges of the pores were rounded off and smoothened. Andfurther, it was found that the walls of the pores were notablysmoothened. The significance of this is that there is less resistance tothe flow of the molten metal through the porous body. The term"consolidation" as used above in this paragraph refers to theapplication of mechanical pressure to the blended mixture of ceramicpowders, resins and pore formers, causing substantially all of the poreformers to be in sufficiently close spatial relationship to each othersuch that pores develop between each cell and at least two (2) adjacentcells, following liquification of the pore formers and sintering.Otherwise, "consolidation" refers to any method by which the mixture ofceramic powder and resin is interposed between and among the poreformers such that pores develop between each cell and at least two (2)adjacent cells. For example, in using pore formers as shown in FIG. 4,the viscosity of the resin/ceramic mixture is reduced to the point whereit can be pour molded by selection of low viscosity resins and/or by theaddition of appropriate plasticizers as will be understood by those withskill in the field.

Once the green body has been formed by way of consolidation, asdescribed above, and the resin has cured, the pore former must beremoved. When a leachable pore former is used, after the resin in thegreen body has cured, the green body is immersed into a solvent todissolve the pore former. For example, when calcium chloride is used asthe pore former, the resin cured green body is immersed in water for asufficient time to dissolve the calcium chloride and leave a porousgreen body. The porous green body is then preferably thoroughly rinsedwith water, to remove traces of calcium chloride, and dried. 0f course,for the pore formers to dissolve and leave, the pores must have beenformed.

When wax or another meltable pore former is used, that pore former mustbe melted out by subjecting the green body to a temperature above themelting point of the pore former material. This can be done as aseparate step or the pore former can be melted out as the green body isgrandually raised to calcining and then sintering temperature as will beexplained hereinafter. When using wax as the pore former, some of thepores are formed by direct contact of the pore formers with each otherafter consolidation, while other pores may be formed by the gas pressurecreated by the volatilization of the hot wax as the temperature of thegreen body is increased toward full sintering temperature.

The shaped, resin-cured green bodies, consisting of the polymer binderand the sinterable ceramic powder, are then fired in a furnace to bothburn off the polymer binder and to sinter or bond the ceramic powderinto a porous ceramic body. As the polymer binder burns off, the ceramicpowder particles move towards each other, densifying as the particlessinter to each other. The overall dimensions of the ceramic bodydecrease as this occurs, producing a dimensional "shrink" and thesintering produce high structural strength and integrity.

Typically, the green body is placed in a furnace which can be controlledto vary the rate of increase in temperature per unit of time. When thesinterable ceramic powder is substantially alumina, the temperature ofthe furnace, for example, beginning at about 150° C., is raised to about600° C. at a rate of about 10° C./hour to 30° C./hour. This stepeffectively calcines the green body. No special atmosphere is needed assintering of alumina will occur in air. At the point where the greenbody has reached 600° C., any meltable pore former should have eithermelted out or vaporized. And most, if not all, of the polymer binder hasburned off at 600° C., and the densification and shrinkage commencesthereafter as the temperature is increased to full sinteringtemperature.

At the point where the substantially alumina green body, substantiallyminus the pore former, has reached, for example, about 600° C., the rateof temperature elevation may be, for example, increased to a range ofabout 200° C./hour to 400° C./hour, and the green body is brought up tofull sintering temperature and held there for a sufficient time forcomplete sintering to occur. For solid state phase sintering of AlcoaA16SG alumina, with a 0.2 wt. percent MgO added, a sintering temperatureof 1550° C. for two hours is generally considered sufficient forsubstantially complete sintering. The sintering produces a mechanicallystrong, self-sustaining body with cells substantially interconnected bypores, with relatively high impact resistance and relativelysubstantially reduced friability in comparison to known ceramic filters.

After a substantially alumina ceramic body has been held at thesintering temperature for a sufficient time to substantially sinter theceramic material, the body must be cooled. It is, of course,economically expedient to cool the body as quickly as possible. On theother hand, care must be taken to cool it slowly enough so that thermalshock and cracking are avoided. Once cooled, the porous ceramic body isready for use.

In mixing the components together for use in injection moldingtechniques, it is preferable to leave out the pore formers from theinitial component mix, thus only mixing together the sinterable ceramicpowder and the polymer binder. The pore formers, however, are loadedinto the cavity of the injection molding apparatus and the sinterableceramic powder/polymer binder mixture is then injected into that cavityto fill the interstitial space between the pore former particles, thussimultaneously admixing the resin/ceramic powder mixture with the poreformers and consolidating those components.

Alternatively, in respect to injection molding, the sinterable ceramicpowders and the polymer binders can be mixed together in such volumesthat the sinterable ceramic powders constitute between 20 and 70 vol.percent of that mixture and, preferably, between 40 and 70 vol. percentthereof. Using thermoplastic resins as polymer binders, these mixturescan be granulated using standard procedures for compounding polymerswith inert fillers. The granules are then injected into the die of aninjection molding apparatus, provided, that the die has first beenloaded with pore formers and the pore formers pressed into a compact bedjust before the injection of the granules. It has been found that thismethod ensures that the pore formers are all in contact with each otherand that all of the interstitial space between the pore formers arefilled with the sinterable ceramic powder/polymer binder mixture. Thepieces are otherwise molded according to standard injection moldingpractice. The sintered porous ceramic bodies thus produced have beenfound to include substantially interconnected cells.

It has also been found that granulated mixtures of sinterable ceramicpowder and thermoplastic resin can be blended with pore formers andpressed together in a standard compression molding apparatus wherein thedie is heated to a temperature selected according to theviscosity/temperature relationship of the particular resin being used.

When thermosetting resins are used, the pieces can be cured in the dieby heating the die following compaction, or the pressed pieces can beremoved directly from the die following pressing and cured in a separatestep. In the case of the use of thermoplastic resins, the pieces areallowed to cool and cure in the die, so as to facilitate their removalfrom that die.

EXAMPLES Example 1

A set of four (4) porous sintered ceramic bodies were fabricated, two ofwhich contained 97 wt. percent Alcoa Al6SG alumina and 3 wt. percentcalcium-alumino-borate glass powder and two of which contained 99.8 wt.percent Alcoa A16SG alumina and 0.2 wt. percent magnesium oxide (MgO).The details of fabrication are as follows:

    ______________________________________                                                          CaCl.sub.2 :                                                         Ceramic: pore for-                                                            Vol. per-                                                                              mer Vol.                                                             cent of  Ceramic,   Density:                                                                             Density:                                           Ceramic  resin and  g/cc   g/cc                                      Sample   plus resin                                                                             pore for-  after  after                                     Code:    mixture  mer        leaching                                                                             sintering                                 ______________________________________                                        A6040    40.5     66         0.67   0.90                                      A7040    40.5     75         0.50   0.66                                      B6040    39.7     66         0.70   0.91                                      B7040    39.7     75         0.48   0.62                                      ______________________________________                                        A6040 and A7040:                                                                         99.8 wt. percent Al.sub.2 O.sub.3, 0.2 wt. percent MgO             B6040 and B7040:                                                                         97.0 wt. percent Al.sub.2 O.sub.3, 3.0 wt. percent                            glass (CaO/Al.sub.2 O.sub.3 /B.sub.2 O.sub.3 = 1/0.79/1.31                    Molar                                                                         Ratio) manufactured by Ferro Corporation                                      and designated as XF 41                                            Pore Former:                                                                             Average diameter, 3.2 mm., CaCl.sub.2, J. T.                                  Baker Desiccant grade                                              Resin:     Epoxy (Epokwick) + Hardener (EpoKwick)                                        both marketed by Beuhler                                           Compaction .sup.˜ 2500 psi @ room temperature                           Pressure:                                                                     Sample Diameter:                                                                         2.25"                                                              Cure:      Room Temperature .sup.˜ 2 hours                              Leaching:  Water @ 90° C. ± 10° C. for .sup.˜ one                 hour                                                               Drying:    Optional                                                           Firing:    150° C.-800° C. @ .sup.˜ 20°                       C./hour                                                                       800° C.-1550° C. @ .sup.˜ 125°                     C./hour                                                                       1550° C. hold for 2 hours                                              1550° C.-20° C. furnace cool, .sup.˜ 6                    hours                                                              ______________________________________                                    

Drying was done between leaching and forming only to enable green bodydensity measurements; drying is not required to make the sintered body.The particular epoxy resin/hardener system was selected as it appearedto bead to a relative greater degree, in contact with CaCl₂ poreformers, than some other epoxy/hardener systems that were considered.

After sintering, the sample pieces were measured to determine theeffects of sintering on the density and sizing of the green bodies asmeasured before firing. The results are as follows:

    ______________________________________                                                         Percent                                                                       change in                                                                     density                                                             Density:  from green                                                          g/cc      body to    Diameter                                                                              Thickness                                 Sample after     sintered   Change: Change:                                   Code:  sintering body       percent percent                                   ______________________________________                                        A6040  0.896     +35.3      -19.9   -17.7                                     A7040  0.662     +32.9      -19.7   -18.1                                     B6040  0.910     +30.2      -18.7   -19.1                                     B7040  0.622     +28.6      -18.9   -19.4                                     ______________________________________                                    

The sintered porous ceramic sample pieces were then examined todetermine the average pore sizes. Those same sample pieces were thentested to determine, firstly, the starting head of molten aluminum thatwas maintained thereby. That is to say, the depth of molten aluminumabove each filter body sample piece was measured. Secondly, thepercentage volume of the filter body, which was filled by moltenaluminum, was determined. The third item which was determined, but priorin time to the molten aluminum tests, was the volume of air per unit oftime that would flow through the filter body at a standard air pressureof 2000 dynes/cm², Re>20, against the entry face of the filter bodysample piece. The results of these tests are as follows:

    ______________________________________                                                                           Air Flow                                                                      Rate                                                Average  Initial          cm.sup.3 /sec                                       Sintered Metal      Vol. %                                                                              at 2000                                    Sample   Pore     Head       Filled                                                                              dynes/                                     Code:    Diameter Depth      by Al cm.sup.2                                   ______________________________________                                        A6040    1.140 mm 11.8 cm    45.6  99                                         A7040    1.245 mm 11.0 cm    40.9  123                                        B6040    0.840 mm 15.6 cm    48.6  74                                         B7040    0.560 mm 24.3 cm    39.3  66                                         ______________________________________                                    

Example 2

A paste was prepared by mixing epoxy resin, a hardener and Al₂ O₃ +0.2%MgO in a ratio such that the ceramic was 40 volume percent of the paste.The epoxy resin and hardener were those that were stated, above, forExample 1, and the alumina (Al₂ O₃) was the Alcoa A16SG material, alsoas stated above for Example 1. This paste was injected into the cavityof an injection molding apparatus after first having packed the 3"diameter die of that injection molding apparatus with +4-6 CaCl₂granules. The injection pressure used was 1600 psi. After substantialinfiltration of the paste into the intersticial space between the packedCaCl₂ granules, the consolidated piece was removed from the die and theresin cured at room temperature. The piece was then leached in water anddried. The piece was then fired according to the firing scheduledefined, above, for Example 1, and cooled. The shrinkage, comparing thegreen body with the sintered body, was 21% for the diameter and 27% forthe thickness. The final density of the sintered body was 1.15 g/cc.

Example 3

A set of two (2) porous sintered ceramic bodies were fabricated whichcontained 99.8 wt. percent Alcoa A16SG alumina and 0.2 wt. percent MgO.The details of the fabrications are as follows:

    ______________________________________                                                         CaCl.sub.2 :                                                        Ceramic:  pore for-                                                           Vol. per- mer Vol. %,                                                         cent of   Ceramic,    Density:                                                                              Density:                                        Ceramic   resin and   g/cc    g/cc                                     Sample plus resin                                                                              pore for-   after   after                                    Code:  mixture   mer mixture leaching                                                                              firing                                   ______________________________________                                        A7060- 40        70          0.51    0.73                                     46-05                                                                         A7060- 40        70          0.55    0.80                                     46-06                                                                         ______________________________________                                        A7060-46-05                                                                            99.8 wt. percent Al.sub.2 O.sub.3, 0.2 wt. percent MgO               A7060-46-06:                                                                  Pore Former:                                                                           -4 + 6 mesh (Tyler) size, CaCl.sub.2                                 Resin:   Standard Oil Co. Silmar S-585 polyester resin with                            0.5 wt. % of Pennwalt Ludicol DDM-9 MEK                                       peroxide added as a curing catalyst                                  Compaction                                                                             A7060-46-05 = 750 psi @ room temperature                             Pressure:                                                                              A7060-46-06 = 1250 psi @ room temperature                            Sample   A7060-46-05 = 2.25" dia × 0.98" thick                          Diameter:                                                                              A7060-46-06 = 2.25" dia × 0.90" thick                          Cure:    .sup.˜ 55° C. for 1/2 hour, die cured                   Leaching:                                                                              Flowing water in a tank @ 70-75° C. for .sup.˜ 1                 hour                                                                          until weight loss had reached a constant value                       Drying:  Optional                                                             Firing:  20° C.-600° C., 29 hrs. @ .sup.˜ 20°               C./hour increase                                                              600° C.-1550° C., 4.75 hrs.                                     @ .sup.˜ 200° C./hour increase                                   1550° C. hold for .sup.˜ 2 hrs.                                  1550° C.-20° C., .sup.˜ 4 hrs., furnace          ______________________________________                                                 cool                                                             

The resin (plus catalyst) was first mixed to uniformity. To this wereadded the Al₂ O₃ and MgO powders. This combination was then mixedsufficiently to ensure homogeneity and complete wetting of the powders.Then the pore former CaCl₂ was added and a third mixing step wasemployed, again to ensure homogeneity and complete admixing of the CaCl₂with the resin/ceramic powder mixture. The mixing times for each of thethree mixing steps was in the range of 1-3 minutes.

The mixture was then removed from the mixer and was loaded into astandard steel die, using a combination of vegetable oil and wax paperas a mold release. The steel dies were then mounted in a Carverhydraulic press and compacted with a dwell time of 10-15 seconds. Thenthe samples were cured in the die at ˜55° C. for one-half hour thenremoved. Following the curing, the samples were placed in a water tank,through which water was flowing, and leached until the weight loss hadreached a constant value, to remove the pore former. Then the sampleswere dried for a sufficient time to evaporate substantially all of thewater from the pores created by leaching. Finally, the dried body wasfired using the Schedule A cycle, described above in this Example.

After sintering, the sample pieces were measured to determine theaffects of sintering on the density and sizing of the green bodies asmeasured before firing. The results are as follows:

    ______________________________________                                                         Percent                                                                       change in                                                                     density                                                             Density:  from green                                                          g/cc      body to    Diameter                                                                              Thickness                                 Sample after     sintered   Change: Change:                                   Code:  sintering body       percent percent                                   ______________________________________                                        A7060- 0.73      +45.0      -22.0   -21.9                                     46-05                                                                         A7060- 0.80      +45.5      -20.5   -22.8                                     46-06                                                                         ______________________________________                                    

The sintered porous sample pieces were then examined to determine theaverage pore sizes. Those same sample pieces were then tested todetermine, firstly, the starting head of molten aluminum that wasmaintained thereby. Secondly, the percentage volume of the filter body,which was filled by molten aluminum as it flowed through that filterbody, was determined. Finally, the aluminum permeability, in lbs/sq.ft./min. was determined by general application of the following formula:##EQU1## The results are as follows:

    ______________________________________                                               Average              Vol. %  Aluminum                                         Sintered  Initial    Filled  Permeability                              Sample Pore      Metal Head by Al   lb/sq. ft.                                Code:  Diameter  Depth      (Porosity)                                                                            min.                                      ______________________________________                                        A7060- 0.031"    6.9"       69       60                                       46-05                                                                         A7060- 0.048"    4.4"       51      190                                       46-06                                                                         ______________________________________                                    

It will be noted that the compaction pressures used in Example 3 weresignificantly reduced from those used in Example 1, this beingattributed to the relatively greater lubricating properties exhibited bythe resin used in Example 3 in comparison to that used in Example 1.

Example 4

A set of two (2) porous sintered ceramic bodies were fabricated using afabrication technique similar to those of Example 3, the onlysignificant difference being that the CaCl₂ pore former was soaked in #2diesel fuel oil prior to incorporation into the resin/ceramicpowder/pore former mixture. The details of the fabrication of these two(2) sample batches are as follows:

    ______________________________________                                                         CaCl.sub.2 :                                                        Ceramic:  pore for-                                                           Vol. per- mer Vol.                                                            cent of   Ceramic,    Density:                                                                              Density:                                        Ceramic   resin and   g/cc    g/cc                                     Sample plus resin                                                                              pore for-   after   after                                    Code:  mixture   mer mixture leaching                                                                              firing                                   ______________________________________                                        OA7060-                                                                              40        70          0.54    0.69                                     46-01                                                                         OA7060-                                                                              40        70          0.63    0.75                                     46-08                                                                         ______________________________________                                    

The compaction pressure used to form green body OA7060-46-01 was 750 psiand to form green body OA7060-46-08 was 1250 psi. The sample size ofOA7060-46-01 was 2.25" diameter×0.90" thick, and the sample size ofOA7060-46-08 was 2.25" diameter×0.86" thick. The firing schedule usedfor the samples of both Example 4 and Example 5, following, is afollows:

60° C.-600° C., 36 hours, ˜15° C. per hour

600° C.-1550° C., 4.75 hours, ˜200° C. per hour

1550° C. hold for 2 hours

1550° C.-20° C., ˜4 hours

After sintering, the sample pieces were measured to determine theaffects of sintering on the density and sizing of the green bodies asmeasured before firing. The results are as follows:

    ______________________________________                                                         Percent                                                                       change in                                                                     density                                                             Density:  from green                                                          g/cc      body to    Diameter                                                                              Thickness                                 Sample after     sintered   Change: Change:                                   Code:  firing    body       Percent Percent                                   ______________________________________                                        OA7060-                                                                              0.69      +26.8      -20.7   -21.3                                     46-01                                                                         OA7060-                                                                              0.75      +19.7      -20.9   -20.5                                     46-08                                                                         ______________________________________                                    

The sintered porous sample pieces were then examined to determine theaverage pore sizes. Those same sample pieces were then tested todetermine, firstly, the starting head of molten aluminum that wasmaintained thereby. Secondly, the percentage volume of the filter body,which was filled by molten aluminum as it flowed through that filterbody, was determined. Finally, the aluminum permeability, in lbs/sq.ft./min. was determined. The results are as follows:

    ______________________________________                                               Average              Vol. %  Aluminum                                         Sintered  Initial    Filled  Permeability                              Sample Pore      Metal Head by Al   lb/sq. ft.                                Code:  Diameter  Depth      (Porosity)                                                                            min.                                      ______________________________________                                        OA7060-                                                                              0.053"    4.0"       63      483                                       46-01                                                                         OA7060-                                                                              0.064"    3.3"       61      542                                       46-08                                                                         ______________________________________                                    

It should be restated and emphasized that the compaction pressurenecessary in Example 4 was significantly lower than that which wasnecessary in Examples 1 and 3, the significant change being that alubricant was used with the pore formers in Example 4 while none wasused in Examples 1 and 3. In Example 3 where the compaction pressureswere parallel, sample-for-sample, with those of Example 4, the aluminumpermeability was greatly increased. The only significant change betweenExample 3 and Example 4 was the addition of a lubricant to the poreformer of Example 4 despite the fact that an epoxy resin, withrelatively lower lubricating characteristics, was used in Example 4.Also, a comparison of the cells and the pores of the samples of Examples1 through 3 with the samples of Example 4 showed a significant increasein the smoothness of the cell walls and the pore walls, and a muchgreater smoothing or rounding of the edges where the cell walls met thepores for the samples of Example 4.

Example 5

A set of two (2) porous sintered ceramic bodies were fabricated whichwere identical in fabrication technique to those of Example 3 exceptthat CaCl₂ was not used as a pore former material. Rather, wax poreformers were prepared from Kindt-Collins #KC210 wax, having a meltingpoint of about 90° C., sufficiently above the 70° C.-75° C. curingtemperature of the resin so as to remain solid during the curing step ofthe process. As with the CaCl₂ pore formers used in previous examples,the wax pore formers were sized at -4+6 mesh (Tyler) size, by standardsieving techniques employing Tyler mesh screening. And, as a consequenceof using wax, instead of CaCl₂, as a pore former, the leaching step wasreplaced with a melting step, following the resin curing step. Theresin-cured green body was heated to about 110° C., somewhat above themelting point of the wax, to liquefy the wax, thus causing it to flowout of the green body, creating the pores. The resin cured green bodywas maintained at this temperature for a time sufficient to permit theweight loss to reach a constant value. In all other respects, thefabrication techniques applied were equivalent to those of Example 1.The details of the fabrication of these two (2) samples are as follows:

    ______________________________________                                                Ceramic:  Wax: Vol.                                                           Vol. per- Percent                                                             cent of   Ceramic,   Density:                                                                              Density:                                         Ceramic   resin, and g/cc    g/cc                                     Sample  plus resin                                                                              pore for-  after wax                                                                             after                                    Code:   mixture   mer mixture                                                                              melt-out                                                                              firing                                   ______________________________________                                        WA7060- 40        70         0.60    0.75                                     46-05                                                                         WA7060- 40        70         0.61    0.75                                     46-06                                                                         ______________________________________                                    

The compaction pressure to form green body WA7060-46-05 was 750 psi andto form green body WA7060-46-06 was 1250 psi. The sample size ofWA7060-46-05 was 2.25" diameter×0.92" thick, and the sample size ofWA7060-46-06 was 2.25" diameter×0.91" thick.

After sintering, the sample pieces were measured to determine theaffects of sintering on the density and sizing of the green bodies incomparison with those measurements before firing. The results are asfollows:

    ______________________________________                                                         Percent                                                                       change in                                                                     density                                                             Density:  from green                                                          g/cc      body to    Diameter                                                                              Thickness                                 Sample after     sintered   Change: Change:                                   Code:  firing    body       percent percent                                   ______________________________________                                        WA7060-                                                                              0.75      +23.9      -20.9   -21.8                                     46-05                                                                         WA7060-                                                                              0.75      +22.5      -21.2   -21.8                                     46-06                                                                         ______________________________________                                    

The sintered porous sample pieces were then examined to determine theaverage pore sizes. Those same sample pieces were then tested todetermine, firstly, the starting head of molten aluminum that wasmaintained thereby. Secondly, the percentage volume of the filter body,which was filled by molten aluminum as it flowed through that filterbody, was determined. Finally, the aluminum permeability, in lbs/sq. ft.min. was determined. The results are as follows:

    ______________________________________                                               Average   Initial   Vol. %   Aluminum                                         Sintered  Metal     Filled   Permeability                              Sample Pore      Head      by Al    lb/sq. ft.                                Code:  Diameter  Depth     (Porosity)                                                                             min.                                      ______________________________________                                        WA7060-                                                                              0.056"    3.75"     71       560                                       46-05                                                                         WA7060-                                                                              0.062"    3.40"     70       625                                       ______________________________________                                    

A significant portion of the cells in this Example were deformed, moreor less in the form of squashed spheres, whereas in previous Examples,the cells were found to be substantially spherical. It is believed thatthe deformed cells were caused by the relative softness of the wax poreformers and the high malleability of wax under pressure.

As will be noted from comparing Examples 3-5, there is a significantincrease in aluminum permeability between Example 3, where a CaCl₂ poreformer was used without a lubricant, and Example 4, where CaCl₂, soakedin #2 diesel fuel oil as a lubricant, was used as a pore former. Alsothere is a significant increase in aluminum permeability between Example4, where lubricated CaCl₂ was used as a pore former and Example 5 whereself-lubricating wax was used. Visual inspection of the cell walls, thepore walls and the pore edges of Examples 3-5 revealed that those ofExamples 4 and 5 appeared much smoother than those of Example 3, withthe pore edges of the samples of Examples 4 and 5 being much morerounded and smoothened than those of Example 3. A comparison of thesamples of Example 4 with Example 5 showed that the samples of Example 5had smoother cell and pore walls and the pore edges of the samples ofExample 5 appeared more rounded and smooth. The pore diameters of thesamples of Example 3 were significantly smaller than those of Examples 4and 5 although the same size of pore formers were used in the samples ofall three of Examples 3-5. Also there is a significant drop in theinitial aluminum metal head depth in comparing the samples of Example 3with those of Examples 4 and 5. In comparing the volume percentage ofthe body filled by molten aluminum; i.e., the effective porosity, of thebodies of Examples 3-5, it is noted that sample A7060-46-05 of Example 3had a greater percentage (69%) of porosity than sample OA7060-46-05 ofExample 4 (63%) while the pore diameter of sample A7060-46-05 was less(0.031") than that of OA7060-46-05 (0.053"); yet the starting head ofA7060-46 -05 was significantly greater than OA7060-46-01 and thealuminum permeability of A7060-46-05 was less than OA7060-46-01. Therewas no significant difference in the fired or sintered density of thesamples of Examples 3-5.

Example 6 Sample Code U7060-46-1

A porous sintered ceramic body was made using urea as a pore former. Forthis sample, Standard Oil Silmar DL-459 polyester resin was used (mixedwith Luperco AMS Hardener as a curing catalyst) along with a ceramicpowder mixture. The ceramic powder mixture was 90 wt. percent AlcoaA16SG Al₂ O₃ plus 10 wt. percent Ferro XF-41 CAB glass frit (CaO/Al₂ O₃/B₂ O₃ =1/0.79/1.31 molar ratio). 60 vol. percent of resin was mixedwith 40 vol. percent of ceramic powder. This mixture was then mixed with-4+6 mesh (Tyler) size urea pore former such that 69 vol. percent of thetotal mixture was pore former. The total mixture was pressed at 750 psiand cured. The urea pore former was melted out at 150° C. The resultantgreen body was fired according to the following schedule:

60° C.-600° C. @˜15° C./hour, 36 hours

600° C.-1200° C. @˜200° C./hour, 3 hours

1200° C. hold for 2 hours

1200° C.-700° C. @˜1000° C./hour, 0.5 hours

700° C.-20° C. @˜340° C./hour, 2 hours

The fired body of Example 6 was not tested with molten metal (aluminum),however, it appeared visually to be quite similar to the samples ofExample 3, above, which were successfully tested with molten aluminum.

Example 7

A set of five (5) porous ceramic bodies were fabricated using #2 dieselfuel oil as a pore former lubricant, and using a vacuum to evacuate theCaCl₂ pore former to enhance soaking by the lubricant. In forming theseporous bodies, the compaction pressure was varied on some to determineif such would have an effect on green body density. The details offabrication are as follows:

    ______________________________________                                                                 Density                                                                              Density                                              Die      Density  g/cc   g/cc   Total                                  Sample Pressure After    after  after  Porosity                               Code:  psi      Pressing leaching                                                                             firing Vol. %                                 ______________________________________                                        8-7A   750      1.74 g/cc                                                                              0.71   0.58   85.0                                   8-7B   750      1.63 g/cc                                                                              0.59   0.55   86.0                                   8-7C   750      1.75 g/cc                                                                              0.66   0.61   84.5                                   8-7D   1500     1.72 g/cc                                                                              0.65   0.55   86.0                                   8-7E   3000     1.81 g/cc                                                                              0.69   0.63   84.0                                   ______________________________________                                    

These samples were formed with lubricated pore formers using thefollowing mixture to make a single batch from which each sample wasformed:

1300 g CaCl₂, -4+6 mesh

480.6 g Alcoa A16SG alumina (Al₂ O₃)

24.62 Ferro XF-41 CAB (calcium alumino borate) glass frit

(CaCO/Al₂ O₃ /B₂ O₃ =1/0.79/1.31 Molar Ratio)

227.5g Standard Oil S-585 Silmar Polyester resin

1.14 g. MEK peroxide, designated DDM-9, manufactured by Ludicol Divisionof Pennwalt

A standard double arm mixer was used to thoroughly blend the mix. An 8"diameter die was used to form sample no. 8-7A while a 2.25" diameter diewas used to form all of the other samples. All of the samples wereleached in water for about 3 hours each. The firing schedule for thesesamples was as follows:

60° C.-600° C. @˜15° C./hour

600° C.-1200° C. @˜200° C./hour

1200° C.--hold for 2 hours

1200° C.-700° C. @˜1000° C./hour

700° C.-20° C. @˜340° C./hour

Samples 8-7C, 8-7D and 8-7E, above, were formed to determine the effecton density by increasing the die pressure; these samples were notfurther tested. It is noted that there was no significant increase indensity after pressing when the die pressure was increased, indicatingthat full pressed density was achieved at about 750 psi.

One of the lubricated pore former samples, sample number 8-7B, was thencompared with sample numbers A7060-46-05 and A7060-46-06, from Example3, which were made with unlubricated pore formers. The results of thesecomparisons are as follows:

    ______________________________________                                                                        Vol. %  Alum-                                                Initial  Pore    Filled by                                                                             inum                                        Die      Metal    Diameter                                                                              Al      Perme-                                Sample                                                                              Pressure Head     (Range ±                                                                           (Porosity)                                                                            ability                               Code: psi      Depth    5%)     min     lb./sq. ft.                           ______________________________________                                        8-7B  750       8.39 cm 1.65 mm 67      1000                                  A7060-                                                                              750      17.53 cm 1.22 mm 69      60                                    46-05                                                                         A7060-                                                                              1250     11.18 cm 1.22 mm 51      190                                   46-06                                                                         ______________________________________                                    

Note that the pore diameter is significantly increased by the use oflubricated pore formers, although the pore former size ranges anddistribution of all three samples were equivalent. The resistance to theflow of molten metal, as measured by the depth of metal head initiallymaintained by the filter body, is significantly decreased for sample8-7B. Even when the compaction pressure is doubled (compare A7060-46-05and A7060-46-06), the initial metal head depth is still significantlygreater when compared to the sample made at lower compaction pressurebut with a lubricated pore former (compare A7060-46-06 and 8-7B).

Example 8

A set of four (4) porous ceramic bodies were fabricated using zirconiumoxide (zirconia, ZrO₂) powder as a base ceramic powder. Yttrium oxide(Y₂ O₃) powder was added to the ZrO₂ powder to produce a ceramic powdermix and the end product ceramic bodies were characterized as partiallystabilized zirconia (PSZ). The ceramic powder mix including 3 mole % ofY₂ O₃ mixed with the balance of ZrO₂ was obtained commercially from ToyoSoda, designated TZ-3Y. The resin used was Standard Oil S-585 Silmar,polyester resin, using Ludicol MEK peroxide as a curing catalyst. Thepore former was +4-6 mesh (Tyler) size CaCl₂ soaked in #2 diesel fueloil. The details of fabrication are as follows:

    ______________________________________                                                         CaCl.sub.2 :                                                        Ceramic:  pore for-                                                           Vol. per- mer Vol. %,                                                         cent of   Ceramic,    Density:                                                                              Density:                                        Ceramic   resin and   g/cc    g/cc                                     Sample plus resin                                                                              pore for-   after   after                                    Code:  mixture   mer mixture leaching                                                                              firing                                   ______________________________________                                        Z-3A   30.7      64.2        0.80    1.42                                     Z-3B   30.7      64.2        0.80    1.46                                     Z-5A   33.1      65.9        0.85    1.63                                     Z-5B   33.1      65.9        0.75    1.39                                     ______________________________________                                    

All of the foregoing ZrO₂ samples were compaction pressed at 1500 psi.All of the green bodies produced were fired according to the followingschedule:

60° C.-600° C., 36 hrs. @˜15° C./hour increase

600° C.-1550° C., 4.75 hrs. @˜200° C./hour increase

1550° C. hold for ˜2 hrs.

1550° C.-20° C., ˜4 hrs., furnace cool

The firing was done in an air atmosphere. All of the mixing leaching andcuring done for the samples of this Example were done according to thespecifications set forth for Example 4, above. The size of the samplesproduced for this Example were all 2.25" diameter with thickness in therange of 0.42"-0.72".

After sintering, the sample pieces were measured to determine theaffects of sintering on the density and sizing of the green bodies asmeasured before firing. The results are as follows:

    ______________________________________                                                         Percent                                                                       change in                                                                     density                                                             Density:  from green                                                          g/cc      body to    Diameter                                                                              Thickness                                 Sample after     sintered   Change: Change:                                   Code:  sintering body       percent percent                                   ______________________________________                                        Z-3A   1.42      +77.9      -27.1   -24.2                                     Z-3B   1.46      +82.3      -27.4   -27.0                                     Z-5A   1.63      +92.9      -27.2   -27.3                                     Z-5B   1.39      +86.6      -26.5   -26.4                                     ______________________________________                                    

The fired bodies of Example 8 were not tested with molten metal(aluminum), however, they appeared visually to be quite similar to thesamples of Example 4, above, which were successfully tested with moltenaluminum. In addition, it is believed that because of the ability of PSZto withstand significantly higher temperatures of molten metals beyondthat of molten aluminum, the PSZ samples of Example 8 may be used tofilter molten copper, brass, bronze, and steels (both mild steels andstainless steels).

One factor that appears to remain consistent in regard to all of thesamples tested and analyzed in the foregoing examples is that the volumeof the cells, created in the porous ceramic bodies, is directlyproportional to the volume of the pore formers initially included in theconsolidated mixtures. In all cases, the volume is reduced but only bythe shrink factor associated with sintering which also affects all otherdimensions of the porous ceramic body in comparison to the correspondinggreen body before sintering. Also, another factor that appears to remainconsistent throughout those samples is that the shape of the pores ispredominantly round. That is to say that more than 50% of the pores havea rounded shape generally approaching the shape of a hollow cylindricalsection, as distinguished from being polyhedral in shape while asubstantial (more than 25%) additional portion of those pores which arenot rounded in shape, are generally oval or elliptical in cross section,as distinguished from having distinct polygon cross sections. Relativelyfew of the pores, in comparison, are generally irregular in shape suchthat distinct polyhedrons are formed thereby.

Several samples, similar to the foregoing samples, but which were nottested with molten metal, were tested to determine strength. ,Twospecific types of standard tests were performed to determine,respectively, Modulus of Rupture (MOR) and Modulus of Bending (MOB). Atroom temperature, the samples tested all exhibited an MOR within therange of 200-260 psi, and at elevated temperature (1200° F.) an MORwithin the range of 110-160 psi. At room temperature, the samples testedall exhibited an MOB within the range of 5.5×10⁴ -7.2×10⁴ psi, and atelevated temperature (1200° F.) an MOB within the range of 4.2×10⁴-4.7×10⁴ psi. All of the samples so tested for strength were fabricatedaccording to the method stated in Example 5 above except that the volumepercent of the ceramic powder used to form the ceramic plus resinmixture was 45 vol. % and the volume percent of the wax pore formersused to form the overall mixture, including pore formers plus ceramicplus resin was 75 vol. %. Based on the foregoing, the conclusion reachedwas that the porous ceramic bodies of the present invention hadsufficiently high enough structural strength and integrity to qualifythem as acceptable molten metal filters.

Referring to FIG. 3, there is shown a schematic representation of amicrograph of a cross section of a porous ceramic body 25 made using Al₂O₃ as the ceramic material and wax as the pore former. The degree ofassimilated magnification relative to actual size is about 6×. Pores 27seen as voids in their top view alignment shown in FIG. 3, arepredominantly round in shape while some tend toward being elliptical oroval shaped pores 29. In this embodiment, generally spherically shapedwax pore formers are used, thus many of the cells 31 are spherical inshape while others take the form of squashed spherical cells 33 asexplained previously. Generally horizontally arranged pores 35interconnect the cells 31, 33. Substantially all of the cells areinterconnected to at least two (2) other cells. Also, the top view pores27, 29 interconnect cells beneath (not shown) to those that are shown inFIG. 3. Finally, separating the cells is a dense, substantially solid(void-free) sintered ceramic matrix 37 which, except for theinterconnection of the pores 27, 29, 35, substantially completely fillsthe interstices between the cells 31, 33. This ceramic matrix 37 isdepicted by the hatched sections of FIG. 3.

Referring to FIG. 3, the diameter of the pores 27 is about 1.27 mm(based on a magnification of about 6×). The Diameter of the sphericalcells 31 is about 3.175 mm based on the same magnification. The diameterof the squashed spherical cells 33 averages about 2.82 mm based on thesame magnification. Accordingly, the ratio of the sizes of the pores 27to the sizes of the cells 31, 33 is within the range of about 0.40--.45.This particular ratio represents the preferred embodiment of the presentinvention; this ratio has been found to produce optimum filterperformance.

In FIG. 3, substantially all of the pore edges 39 which form thediscrete areas where there is a transition from a cell wall 40 to a porewall 38 are rounded off or smoothed such that there are no sharp cornersor rough edges to cause the smooth flow of molten metal to be hinderedor restricted. Surprisingly, however, the exogenous intermetallicsubstances, and other impurities including slag and dross, are trappedwithin the cells 31 and 33 during the flow of molten metal through theceramic body 25 with relatively insubstantial amounts passingtherethrough. Those exogenous intermetallic substances were found to betrapped substantially uniformly within the cells 31 and 33. Thus, thosenon-metallic substances were found to be substantially uniformlydistributed through the cross section or thickness 13 of the ceramicbody 11 as viewed in FIG. 2 extending from entry face 21 to exit face 23whereas, in most prior art filter media, the exogenous intermetallicsubstances, after molten metal flow, are found to be collected at orclose to the entry face thereof.

According to the Patent Statutes, the best mode and preferred embodimentof the present invention have been described. However, the scope of thepresent invention is not limited thereto, but rather, is defined by theappended claims.

What is claimed is:
 1. A porous ceramic body of high strength andintegrity, comprising:a substantially continuous ceramic matrixcharacterized by the absence of filamentary pores; a plurality ofvariously sized cells randomly distributed and interspersed throughoutsaid ceramic matrix, said ceramic matrix substantially separating thecells and filling the interstices between and among said cells; and aplurality of non-filamentary pores interconnecting said cells,predominately each of said cells being interconnected by said pores toat least two (2) other of said cells, the walls of said cells and saidpores being predominately smooth, the edges of said pores formingdiscrete areas of transition between said cell walls and said porewalls, said edges being rounded and smooth, the average cross-sectionalarea of said pores being predominately smaller in size than the averagecross-sectional area of said cells.
 2. The invention of claim 1 whereinsaid ceramic matrix is selected from the group consisting of Al₂ O₃,SiC, TiB₂, B₄, Si₃ N₄, SiAlON and PSZ.
 3. The invention of claim 1wherein said porous ceramic body is a filter medium.
 4. The invention ofclaim 1 wherein said cells are predominantly spherical in shape.
 5. Theinvention of claim 1 wherein said cells are predominantly non-sphericalin shape.
 6. A porous ceramic body of high strength and integrityproduced by a method comprising the steps of:forming a substantiallyhomogenous mixture of curable resin and sinterable ceramic powder;admixing said mixture of said resin and said powder to a plurality ofpore formers to form an admixture; consolidating said admixture suchthat substantially all voids are eliminated; curing said resin withinsaid admixture to form a green body; removing said pore formers fromsaid green body; and after said pore formers are removed from said greenbody, firing said green body to sintering temperature and sintering saidpowder to form said porous ceramic body of high strength and integrity,said porous ceramic body including a plurality of non-filamentary pores;andwherein said resin and said pore formers are selected such thatwetability of said resin and said pore formers tends to cause said resinto bead when in contact with the surfaces of said pore formers beforesaid resin is cured.
 7. The invention of claim 6 wherein said sinterableceramic powder is selected from the group consisting of Al₂ O₃, SiC,TiB₂, B₄ C, Si₃ N₄, SiAlON and PSZ.
 8. The invention of claim 6 whereinsaid curable resin is selected form the group consisting of polyester,epoxy, polyethylene, polypropylene, phenolic and polyvinylchloride. 9.The invention of claim 6 wherein said curable resin is mixed with aplasticizer before it is formed into said mixture of said curable resinand said sinterable ceramic powder.
 10. The invention of claim 6 whereinsaid curable resin is mixed with a lubricant before it is formed intosaid mixture of said curable resin and said sinterable ceramic powder.11. The invention of claim 6 wherein said curable resin is mixed with acuring agent before it is formed into said mixture of said curable resinand said sinterable ceramic powder.
 12. The invention of claim 6 whereinsaid curable resin has lubricating properties.
 13. The invention ofclaim 6 wherein said curable resin is a thermosetting resin.
 14. Theinvention of claim 6 wherein said curable resin is a thermoplasticresin.
 15. The invention of claim 6 wherein said pore formers comprisewax.
 16. The invention of claim 6 wherein said pore formers compriseCaCl₂.
 17. The invention of claim 6 wherein said pore formers havelubricating properties.
 18. The invention of claim 6 wherein thesurfaces of said pore formers are coated with oil before being admixedwith said mixture of said resin and said powder.
 19. The invention ofclaim 6 wherein said pore formers are predominantly spherical in shape.20. The invention of claim 6 wherein said pore formers are predominantlynon-spherical in shape.
 21. The invention of claim 6 wherein said poreformers are removed by liquification.
 22. The invention of claim 6wherein said pore formers are removed by leaching.
 23. The invention ofclaim 6 wherein said pore formers are removed by melting.
 24. Theinvention of claim 6 wherein said pore formers are removed bypyrolization.
 25. The invention of claim 6 wherein said pore formers areremoved by sublimation.
 26. The invention of claim 6 whereinpredominantly each of said pore formers within said green body are incontact with at least two other of said pore formers.
 27. The inventionof claim 6 wherein said step of consolidating comprises pressurecompacting.
 28. The invention of claim 6 wherein said step ofconsolidating comprises pour forming.
 29. The invention of claim 6wherein said step of consolidating comprises injection molding.
 30. Theinvention of claim 6 wherein said step of consolidating comprises slipcasting.
 31. The invention of claim 6 wherein said porous ceramic bodyis a filter medium.
 32. The invention of claim 6 wherein said porousceramic body comprises:a substantially continuous and solid ceramicmatrix; a plurality of cells distributed and interspersed throughoutsaid ceramic matrix, said ceramic matrix substantially separating thecells and filling the interstices between and among said cells; and saidplurality of non-filamentary pores interconnecting said cells,predominately each of said cells being interconnected by said pores toat least two (2) other of such cells, the walls of said cells and saidpores being predominately smooth, the edges of said pores formingdiscrete areas of transition between said cell walls and said porewalls, said edges being rounded and smooth, the average cross-sectionalarea of said pores being predominately smaller in size than the averagecross-sectional area of said cells.
 33. A porous ceramic body of highstrength and integrity formed by removing the pore formers from, andthen sintering, a green body which comprises a consolidation of apredominately homogenous mixture of a curable resin with a sinterableceramic powder which is intermixed with a plurality of said poreformers, the materials of said resin and said pore formers beingselected such that the wetability of said rein in respect to said poreformers tends to cause said resin to bead when in contact with thesurfaces of said pore formers before said resin is cured, said resin insaid consolidation being curable and said pore formers in saidconsolidation being removable after said resin in said green body hasbeen cured.